linux-kernel - Re: [PATCH V3] scsi: core: use blk_mq_requeue_request in __scsi_queue

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Date:   Fri, 2 Mar 2018 16:31:33 +0000
From:   Bart Van Assche <Bart.VanAssche@....com>
To:     "jejb@...ux.vnet.ibm.com" <jejb@...ux.vnet.ibm.com>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
        "jianchao.w.wang@...cle.com" <jianchao.w.wang@...cle.com>
CC:     "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "hch@....de" <hch@....de>
Subject: Re: [PATCH V3] scsi: core: use blk_mq_requeue_request in
 __scsi_queue_insert

On Fri, 2018-03-02 at 11:31 +0800, Jianchao Wang wrote:
> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> index a86df9c..d2f1838 100644
> --- a/drivers/scsi/scsi_lib.c
> +++ b/drivers/scsi/scsi_lib.c
> @@ -191,7 +191,13 @@ static void __scsi_queue_insert(struct scsi_cmnd *cmd, int reason, bool unbusy)
>  	 */
>  	cmd->result = 0;
>  	if (q->mq_ops) {
> -		scsi_mq_requeue_cmd(cmd);
> +		/*
> +		 * scsi_device.sdev_gendev will be get every time in .get_budget and
> +		 * be put in scsi_end_request. Hence we need to put the reference
> +		 * here when we decide to requeue request.
> +		 */
> +		blk_mq_requeue_request(cmd->request, true);
> +		put_device(&device->sdev_gendev);
>  		return;
>  	}
>  	spin_lock_irqsave(q->queue_lock, flags);

Please consider to change the new comment into something like the following:
"Before a SCSI command is dispatched, get_device(&sdev->sdev_gendev) is called
and the host, target and device busy counters are increased. Since requeuing a
request causes these actions to be repeated and since scsi_device_unbusy() has
already been called, put_device(&device->sdev_gendev) must still be called. Call
put_device() after blk_mq_requeue_request() to avoid that removal of the SCSI
device can start before requeueing has happened."

Thanks,

Bart.